Capacitors

[electricfarmer]

I would like to know the difference between round and oval run capacitors. The oval ones cost more, what is the difference? Also, why are start capacitors "split Rated"? Ex. 189- 227 MFD, and a run capacitor only has one stated value, usually +- 5%. Any help appreciated. THanks

[Al Skierkiewicz]

Even though this is not a robot related question, here is an answer.
There is no difference between the oval or the round motor start capacitors. It is simply the manufacturer, the desire of the motor designer or a combination of the two. A 100 mfd cap is the same value regardless of the case style. In general, the oval caps in the past have been of an oil filled variety while the round ones are standard electrolytic. A "split cap" has two capacitors inside and generally, these are used in multispeed motors like a furnace blower or in a motor design that uses one capacitor for start and another for run. In either case, there is a switch inside the motor that switches these based on motor speed.
AC motors used on single phase have very little start torque. In order to get good starting torque an AC motor needs at least two phases. However, there is a way to simulate a second phase by switching a capacitor in series with another winding. When at speed, an AC motor achieves maximum power based on it's design. A simple method to make this switch is to have a centrifugal mechanism attached to the motor shaft internal to the motor. When the motor shaft reaches a predesigned speed, the weights in the mechanism are thrown out by the rotating forces which then disengages the capacitor and switches to the run winding. Three phase motors do not use/need a switch or start capacitor but as you know, three phase power is rarely supplied to small businesses or residential buildings.